Venous flow stimulator

ABSTRACT

Apparatus for automatically inflating and deflating, in a predetermined pressure cycle, at least one double-walled pneumatic boot fitted to the lower limb of a patient undergoing surgery.

United States Patent McGrath July 29, 1975 541 VENOUS FLOW STIMULATOR2,140,898 12/1938 Collens et al. 128/24 R r 3,179,106 4/1965 Meredith128/64 X 1 lnvemo" McGrafl" Harm, 3,303,841 2/1967 Dennis 128/24 REngland 3,390,674 7/1968 Jones l28/DIG. 10 Assignee: The British yg p y3,527,207 9/l970 Gottfried l28/24 R Limited, London, England PrimaryExaminer-Lawrence W. Trapp [2,2] Flled 1973 Attorney, Agent, orFirmDennison, Dennison, [21'] Appl. No.2 424,813 Townshend & Meserole 52us. c1. 128/24 R 57 ABSTRACT [51] Int. Cl A6lh l/00 1 Field 05 Search28/ Apparatus for automatically inflating and deflating, in

l28/D lG. 20, 325- a predetermined pressure cycle, at least onedoublewalled pneumatic boot fitted to the lower limb of a pa- [56]References Cited tient undergoing surgery.

UNITED STATES PATENTS 12 Cl 4 D 2,071,215 2/1937 Petersen 128/24 R guns.1 Z ,5? 72 if 24 28 L 7 Nt 45 PATENTEI] JUL 2 91975 l VENOUS FLOWSTIMULATOR This application relates to a venous flow Stimulator, bywhich is meant a device used in some surgical opera tions to applypulsating pneumatic pressure to the legs of a patient while undergoingsurgery, in order to prevent the occurrence of post-operative deepvenous thrombosis.

A known venous flow stimulator includes an electrically driven pumpconnected to one ora pair of inflatable pressure applicators (orpneumaticboots) adapted to be positioned around the patients legs andfeet. Pressure is applied (and released periodically) to the calfmuscles of the legs of the patient by means of the boots so as toprevent stasis of blood flow in the deep veins of the legs. This reducesthe risk of deep venous thrombosis occurring.

It is necessary to use the known form of venous flow stimulator to treata patient throughout the preoperative, operative and postoperativeperiod, treatment ceasing when the patient is ready to get out of bed.Each patient requires a machine for a relatively long period, andpost-operatively the treatment may cause him discomfort andapprehension. Because the treatment is prolonged a considerable numberof machines have to be employed and surgeons are reluctant to bring theminto general use.

It is the aim of the present invention to provide an improved venousflow stimulator giving more effective treatment to enable the period oftreatment to be reduced to that taken up by the surgical operationitself,

so that the venous flow stimulator can therefore be essentially anapparatus for use only in the operating theatre.

According to the present invention there is provided a venous flowstimulator which is as claimed in the appended claims. r

The venous flow stimulator of the present invention will now bedescribed by way of example with reference to the accompanying drawings,in which:

FIG. 1' is a schema of one form of flow stimulator of the presentinvention;

FIG. 2 is a diagrammatic view of the stimulator in position during asurgical operation;

FIG. 3 is a graph of voltage v. time for the electrical pulsescontrolling operation of the stimulator, and

FIG. 4 is a graph of pressure v. time for the pneumatic pulses suppliedby the stimulator to the boots encasing the patients legs.

The venous flow stimulator shown in FIG. 1 includes a source 2 ofpressurised gas usually in the form of the portable cylinder 4 shownin'FlG. 2 leading through a solenoid-operated valve 6 to a flowregulator 8 (for example, of the type described in our UK patentspecification No. 1,039,528). The flow regulator 8 leads through conduit10, to a device 12 adaptedto generate pulses of pressurisedgas. I

The device 12 has a gas supply chamber 14 of which an outlet 16 iscontrolled by a rocker valve 18'. One wall of chamber 14 is formed by adiaphragm 20 carrying a permanent magnet 22. The device 12 also has acontrol chamber 24 formed by bellows 26, of which the end wall carries asecond permanent magnet 28 and a conduit 30 leading to an adjustablebleed valve 32. Extending from the device 12 is a conduit 34in which ispositioned an-injector 36. The injector is adapted to receive gas underhigh pressure from conduit 34 and dilute it substantially with air fromthe atmospherefin order to'produce larger volumes of gas at lowerpressure suitable for pressurising both boots 38 fitted to the feet andlegs of the patient.

Downstream of the injector 36 the conduit 34 leads to two furtherconduits 40 and 42. The conduit 40 leads to the inlet of a pneumaticallyoperated exhaust valve 44, while the conduit 42 leads to apressure-relief valve 46 and to the interior of chamber 24 through anonretu rn valve 48. I

Operation of the device [2 is governed by the pivotal position of rocker18, which is in turn controlled bythe relative positions of magents 22and 28. The rocker I8 is of a soft magnetic material, such as Swedishiron. so that the rocker is able to be pivoted about its fulcrum whenone or other of the magnets becomes dominant. As magnet 22 is positionedon the stationary diaphragm 20, the force it applies on rocket 18 in thevalve-closing direction is substantially constant. Because the magnet 28is positioned on the movable end wall of bellows 26, the magnet 28 ismovable between two limit positions, in one of which its magnetic fieldovercomes that exerted on rocker 18 by magnet 22 and forces the rockerto pivot in the valve-opening direction. In the other limit position themagnetic field produced by magnet 28 falls below that exerted on rocket18 by magnet 22 thus causing the rocker to be moved in the valveclosingdirection.

A conduit 50 is pressurised from source 2 and leads to a pressure switch52 which, when closed,.causes a battery 54 to be connected across acapacitor C and across the input terminals of an electronic timer 56having three outlets (labelled X, Y and Z) which are connected torespective terminals on a solenoid 58 controlling operation of valve 6.

The detailed circuitry of timer 56 is not described in thisspecification, but it is based on the use of digital integratedcircuitry of the COS/MOS type. In particular, although this is not shownin the drawings or described herein in any greater detail, the timerincludes a square-wave oscillator adapted to be energised by battery 54.The output of the oscillator is fed through a pulse-shaping circuit to asolid state switching circuit. Both the oscillator and pulse-shaperconsist of two input, quadded NOR gates, which are availablecommercially, one form thereof being sold by RCA as an item in theirCOS/MOS CD4000A series of integrated circuits. The various components ofthese integrated circuits are interconnected through suitably ratedelectrical components to derive an output having the characteristicsshown in FIG. 3 of the accompanying drawings. in a typical output, thereis a positive-going pulse of 9 V amplitude and lasting 50 ms, followedafter an interval of about 10 s by a negative-going pulse of the sameamplitude and duration. At a period of between --120s after theoccurrence of the negative-going :and some circuit is kept static untilarrival of the next positive pulse. Thus the risk is avoided of havingthe boots inflated during the long resting phase, and kept deflatedduring the intended inflation phase, as could happen were both the setand reset pulses were of the same polarity.

As already mentioned, these pulses are applied to the respectiveterminals of solenoid 58, the setting pulses serving to switch the valve6 into the position in which the source 2 applies a continuous stream ofgas under pressure to device 12 through pressure regulator 8. When areset pulse is received, the valve is returned to the illustratedposition in which no further pressurised gasjis supplied to regulator 8,the conduit upstream thereof being vented to atmosphere.

The function of capacitor C is to act as an energystoring device toensure that the current drain on battery 54 is reduced to the minimumconstant with reliable operation, while at the same time lengthening thelife of battery 54 so that it needs to be replaced only at infrequentintervals. The timing circuit 56 is such that when a set or reset pulseis applied by it to solenoid 58, most of the energy for the pulse isderived from capacitor'C. Thus ensures that the capacitor C is chargedby battery 54 when no pulses are being produced by the timer, thusconverting a substantially constant current drain on battery 54 intointermittent current pulses of relatively high amplitude. The functionof pressure switch 52 is to disconnect the battery 54, and disable thetimer circuit, when there is insufficient gas pressure in the inlet tothe pneumatic circuit to operate the pneumatic boots 38.

The boots 38 themselves are already known, and so will not be describedin great detail in this specification. The boots are double-walled, andat least the inner wall is made of a flexible plastics material which isable to takeup the contours of the patients lower leg and foot. Airunder pressure is introduced into the space between the two walls. Whenthe air of other gas is first introduced, it causes the outer wall todistend until it reaches its final shape, after which the furtherincrease in pressure forces the inner wall more and more firmly againstthe patients limb. This pressure is transmitted to the patients veinsand other blood vessels, causing them to dilate.

, This contraction expels blood from the vessels in the directiondictated by the usual valving arrangements forming part of the bodysvascular system. When a desired maximum pressure has been reached thepneumatic pressure in the boot is released, as by venting to atmospherethe space between the two walls of the boot. This reduction in pressureallows fresh blood to be pumped by the heart into the limbs and bloodvessels. The cyclic compression and expansion applied by the bootssimulates the massaging of the legs blood vessels which is normallyapplied by the calf muscles when the patient is standing or walking, butwhich action is inhibited or stopped when the patient is anaesthetizedand is in a lying position.

The present invention therefore increases both the peak femoral veinflow and pulsatility, leading to the same mean mass transfer of blood aswhen the body is working normally.

In accordance with the present invention, it has been found that theeffectiveness of the pneumatic boots is related closely to the patternof the pressure cycle to which,they are subjected. In particular, it hasbeen found that amongst the critical factors are: the rate at whichpressurizing gas is applied to the pneumatic boots; the maximum whichthis pressure reaches; the speed with which the pressure is reduced, andthe timing between successive pressurization cycles. Thesecharacteristics are indicated diagrammatically in FIG. 4. Before theseare discussed in any further detail, the operation of the venous flowstimulator will, be described in further detail.

In the initial, unenergized, state of the stimulator, the bellows 26 isin its deflated condition, being biased to that position by acompression spring (not shown). The detailed construction and operationof the bellows is as described in our UK patent specification No. 866758, and so will not be described in greater detail herein. In thisposition the magnet 28 is more effective than is magnet 22, thus biasingthe rocker 18 open so that the inlet 16 of conduit 34 is incommunication with chamber 14. The exhaust valve 44 is open, allowingthe interior of both boots 38 to vent to atmosphere.

When it is desired to start operation of the apparatus, the gas underpressure is applied to pressure switch 52 and to valve 6 from a suitablesource, such as a cylinder of compressed air, nitrogen or oxygen. Theuse of oxygen might seem surprising, in view of its higher cost but ithas been found that oxygen is usually more readily available inhospitals than the other gases and so is more convenient to use despiteits higher cost. The possible or preferred use of oxygen means that noneof the valves in the apparatus, at least on the high-pressure sidethereof, can be lubricated, because oxygen can react explosively withsome lubricants under certain conditions. The valves are thereforedesigned with the use of oxygen in mind. I

Closure of pressure switch 52 causes the timer 56 to send out a setpulse to solenoid 58. This switches over valve 6 to the position inwhich gas under pressure flows'through the pressure regulator 8 and intothe interior of chamber 14. From there, the rocker 18 being open, itpasses into injector 36, is diluted with atmospheric air and passes intothe operating volume of the pneumatic boots 38. As soon as asuperatmospheric pressure is generated in conduit 34 this causes theexhaust valve 44 to be switched over so that the conduit is isolatedfrom the atmosphere. By virtue of conduits 34 and 42, the pressure inthe interior of the boots 38 is transmitted to the pressure-relief valve46 and to the interior of control chamber 24 of device 12. As thispressure is greater than that needed to overcome the force of thebiasing spring, the chamber 24 starts to increase in volume, althoughgas is bled away from chamber 24 at a rate determined by the setting ofneedle valve 32. Under normal conditions of operation, the valve 32bleeds gas away from chamber 24 at a rate much lower than that at whichgas enters the chamber from conduit 42, so that the bellows 26 aredistended ata chosen rate. As the magnet 28 is moved away from rocker 18by this distention of the bellows, there comes a point when the field ofmagnet 28 is less effective than that of magnet 22, and the rocker valvecloses. This is arranged to take place when the pressure in the interiorof boots 38 has reached a desired maximum. According to the teaching ofthe present invention, a desired rate of pressurization of the boots issuch that the operating pressure therein should increase at the rate of8 mm of mercury (Hg) per second, and the desired maximum value should beof the order of 50 mm Hg, all the pressures being measured aboveatmospheric. Thus the boots would take about 6-7 seconds to inflate.

When the rocker 18 has switched over, this causes the pressure inchamber 14 to increase until the arrival of the next reset pulse atsolenoid 58, but this pressure is not passed on-- the pneumatic boots,because of the closure of the rocker valve 18. When the reset pulse doesarrive, it switches over valve 6 and enables chamber 14 to be vented toatmosphere With closure of rocker valve 18, the reduction of pressure inconduit 34 allows valve 44 to open, thus venting to atmosphere theinterior of both boots 38. lt is envisaged that this venting would takeplace under the natural compliance of the system but it could beassisted by a partial-vacuum device (not shown) adapted to apply ameasure of vacuum to the interior of the boot to increase the rate atwhich they are deflated. This reduction in pressure closes thenon-return valve 48, but the chamber 24 continues to be vented throughvalve 32, thus allowing magnet 28 to move towards the position in whichit is effective to open rocker 28. However, the rate-of-return of magnet28 is governed so that by the time it is effective to open the rockervalve, the inlet valve 6 has received the reset pulse and closed.

The stimulator then stays in its rest position awaiting the arrival ofthe next set pulse, which occurs some 100 to 120 seconds after thepreceding reset pulse. This interval is usually set by the manufactureof the stimulator, or it may be under the control of the anaesthetist orother person in the operating theatre.

It has been found that the stimulation of the bloodpumping movement ofthe calf muscles is so effective that the venous flow stimulator of thepresent invention need be used only during the surgical operation.

It is within the purview of the present invention to apply the pressurepulses to the two boots alternately. This would require a modificationin the outlet circuit of the stimulator, involving principally theaddition of a pneumatic flip-flop valve, and to altering the timingcircuit of the stimulator so that it generates pulses of the same shapebut at twice the frequency described above. This would ensure thatalternate pulses would be applied to each of the boots, so that each ofthe boots would receive exactly the same cycle of pulses as describedabove, but with the two cycles being out of phase with each other.However, so far it has been found by experiments that there is nosignificant advantage to the patient in having alternate pressurizationof the boots, and so the additional cost and complication of providingthis option are not usually justified.

What we claim is:

1. An apparatus useable in conjunction with a source or pressurized gasfor stimulating venous blood flow in the legs of patients undergoingsurgery, said apparatus including at least one double-walled pneumaticboot positioned on and completely enclosing a patients leg and foot;said boot having an operating space defined between the walls thereof; asupply valve operative to control the supply of gas at a suitablepressure from a pressurized source to the operating space; meansresponsive to the gas pressure in the operating space for closing thesupply valve when the pressure reaches a chosen maximum valve; a timermeans adapted to produce a series of successive set and reset pulses,and an inlet valve means, controlled by the timer, for supplying gas tothe supply valve upon receipt of a set pulse, and for discontinuing thesupply of gas upon receipt of a reset pulse.

2. A venous flow stimulator as claimed in claim 1, including injectormeans positioned downstream of the supply valve between the supply valveand boot for receiving high-pressure gas from the open supply valve andfor diluting it with atmospheric air thereby producing a larger volumeof gas at lower pressure for feeding to the pneumatic boot as long asthe supply valve is open.

3. A venous flow stimulator as claimed in claim I, including means forenergizing the timer only when the pressure of the supply of gas to thesaid inlet valve is above a chosen minimum.

4. A venous flow stimulator as claimed in claim 3, in which the timer iselectronic and is composed of units of integrated circuitry.

5. A venous flow stimulator as claimed in claim I, in which the saidsupply valve includes a magnetic rocker valve and two permanent magnetsmounted for relative movement, said rocker valve being operated byrelative movement of said magnets.

6. A venous flow stimulator as claimed in claim 5, in which the saidsupply valve includes a bellows having a portion thereof movablerelative to said rocker valve, said movable portion carrying one of thesaid two permanent magnets for movement thereof toward and away fromactuating relationship with the rocker valve.

7. A venous flow stimulator as claimed in claim 6, in-

cluding an adjustable bleed valve in the bellows, said bleed valveventing the bellows to atmosphere at a desired rate.

8. A venous flow stimulator as claimed in claim 1, including an exhaustvalve biased to the open position, said exhaust valve being operativelycontrolled by the outlet pressure of the gas from the supply valve in amanner whereby when the outlet pressure exceeds a chosen valve theexhaust will close, the inlet of the exhaust valve being incommunication with the operating space of the boot for a selectiveventing thereof.

9. A venous flow stimulator as claimed in claim 4, in which the timerproduces set and reset pulses of opposite polarity.

10. A venous flow stimulator as claimed in claim 9, in which theintervals between successive set and reset pulses is approximately 10seconds, and between successive reset and set pulses is approximatelyseconds.

11. A venous flow stimulator as claimed in claim 10, in which shuntedacross input terminals of the timer is a capacitor adapted to bedischarged through the timer to contribute energy to the pulses, and tobe charged in the intervals between the pulses.

12. The method of stimulating venous blood flow in the leg of a patientundergoing surgery comprising intermittently forcing blood from theveins of the leg by an intermittent pressurization of a leg and footencasing pneumatic boot at an increasing rate of 8 mm of mercury persecond to a maximum on the order of 50 mm of mercury, and subsequentlyreleasing the pressure.

1. An apparatus useable in conjunction with a source or pressurized gasfor stimulating venous blood flow in the legs of patients undergoingsurgery, said apparatus including at least one double-walled pneumaticboot positioned on and completely enclosing a patient''s leg and foot;said boot having an operating space defined between the walls thereof; asupply valve operative to control the supply of gas at a suitablepressure from a pressurized source to the operating space; meansresponsive to the gas pressure in the operating space for closing thesupply valve when the pressure reaches a chosen maximum valve; a timermeans adapted to produce a series of successive set and reset pulses,and an inlet valve means, controlled by the timer, for supplying gas tothe supply valve upon receipt of a set pulse, and for discontinuing thesupply of gas upon receipt of a reset pulse.
 2. A venous flow stimulatoras claimed in claim 1, including injector means positioned downstream ofthe supply valve between the supply valve and boot for receivinghigh-pressure gas from the open supply valve and for diluting it withatmospheric air thereby producing a larger volume of gas at lowerpressure for feeding to the pneumatic boot as long as the supply valveis open.
 3. A venous flow stimulator as claimed in claim 1, includingmeans for energizing the timer only when the pressure of the supply ofgas to the said inlet valve is above a chosen minimum.
 4. A venous flowstimulator as claimed in claim 3, in which the timer is electronic andis composed of units of integrated circuitry.
 5. A venous flowstimulator as claimed in claim 1, in which the said supply valveincludes a magnetic rocker valve and two permanent magnets mounted forrelative movement, said rocker valve being operated by relative movementof said magnets.
 6. A venous flow stimulator as claimed in claim 5, inwhich the said supply valve includes a bellows having a portion thereofmovable relative to said rocker valve, said movable portion carrying oneof the said two permanent magnets for movement thereof toward and awayfrom actuating relationship with the rocker valve.
 7. A venous flowstimulator as claimed in claim 6, including an adjustable bleed valve inthe bellows, said bleed valve venting the bellows to atmosphere at adesired rate.
 8. A venous flow stimulator as claimed in claim 1,including an exhaust valve biased to the open position, said exhaustvalve being operatively controlled by the outlet pressure of the gasFrom the supply valve in a manner whereby when the outlet pressureexceeds a chosen valve the exhaust will close, the inlet of the exhaustvalve being in communication with the operating space of the boot for aselective venting thereof.
 9. A venous flow stimulator as claimed inclaim 4, in which the timer produces set and reset pulses of oppositepolarity.
 10. A venous flow stimulator as claimed in claim 9, in whichthe intervals between successive set and reset pulses is approximately10 seconds, and between successive reset and set pulses is approximately100 - 120 seconds.
 11. A venous flow stimulator as claimed in claim 10,in which shunted across input terminals of the timer is a capacitoradapted to be discharged through the timer to contribute energy to thepulses, and to be charged in the intervals between the pulses.
 12. Themethod of stimulating venous blood flow in the leg of a patientundergoing surgery comprising intermittently forcing blood from theveins of the leg by an intermittent pressurization of a leg and footencasing pneumatic boot at an increasing rate of 8 mm of mercury persecond to a maximum on the order of 50 mm of mercury, and subsequentlyreleasing the pressure.